A common method for testing semiconductor integrated circuit chips is to contact a conductor on the chip with a conductive probe so that the voltage on the conductor can be measured. As microelectronic circuit densities increase, the deleterious effects of reactive loading on the circuit by the probe also increase; it also becomes more difficult to make reliable contacts. The U.S. patent of Bloom et al., No. 4,681,449, granted Jul. 21, 1987, and the paper, "100 GHz On-Wafer S-parameter Measurements by Electrooptic Sampling," by R. Majidi-Ahy et al., 1989 IEEE MTT-S Digest, pp. 299-301, are examples of the literature concerning electro-optic sampling which avoid the need for a conductive probe. This technique relies on the proximity of an electro-optic material to the conductor under test, and is particularly applicable to the testing of group III-V semiconductor chips such as gallium arsenide which are inherently electro-optic. The electric field around the conductor extends into the electro-optic material and modulates the polarization of a laser beam that is directed through the electro-optic material. By analyzing the polarization modulations of the exiting laser beam, one can characterize the voltage on the conductor, and thereby test or diagnose the operation of the semiconductor chip. If the semiconductor chip is not of electro-optic material, one can still use this method by placing an electro-optic probe tip sufficiently close to a conductor of the chip to permit modulation of laser light by electric fields extending through the electro-optic material.
The laser used is typically a pulsed laser producing a beam of light pulses having a repetition rate related to the frequency of the signal in the device under test. For example, in the Bloom et al. patent, the electrical signal exciting the device under test is taken from the laser beam used for probing. In the Majidi-Ahy paper, separate excitation sources are used for the laser and for the device under test, and a timing stabilizer is used to coordinate the pulse rate of the laser with the frequency of the signal in the device under test. The paper, "Electro-optic Sampling Measurements of High Speed InP Integrated Circuits," by J. M. Weisenfild, et al. Applied Physics Letters, Vol. 15, No. 19, May 11, 1987, pp. 1310-1312, describes a method that makes use of a mixer for mixing the test signal driving the device with the signal driving the laser so as to provide a difference frequency that can be used as a reference for analyzing the test data. This requires that the frequency of the laser pulses be faily close to the device excitation frequency, which often is not true, particularly if one desires to test extremely high frequency circuits. For example, a practical pulse rate for the laser may be one gigahertz, while the device may require testing at a frequency of ten gigahertz. Any scheme using separate sources for exciting the laser and the device under test runs the risk of phase drift during the testing, which may lead to inaccurate results. We have found that phase drifts as rapid as three degrees per minute are common even with the best equipment available on the market. Accordingly, there is a continuing need for non-invasive probing techniques that give accurate results, particularly in terms of phase, and which are consistent with high frequency testing, particularly at high microwave frequencies.